ND filter and Herschel wedge comparison

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Alun_H
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ND filter and Herschel wedge comparison

Post by Alun_H »

Had chance to grab a couple of scans in the Hydrogen alpha wavelength earlier this afternoon using the 72ED and Sol'Ex firstly with the Hoya PRO ND16 full aperture filter and then followed by removing the filter and putting the Herschel wedge in place,both used the same slew speed to force the scan and wedge image had a slight increase in exposure and gain and there is a distinctive difference between the two images with the filter softening the detail. I have left the images as they came out of the reconstruction software,the only manipulation was reducing the size to keep the file size down. I reckon from now on I will remain with the wedge although,as I stated in an earlier thread,I am not a fan of imaging with any diagonal.

Alun

2022-11-13-1240_9-U-L-Sun_QHY5III178M_shift=0_clahe.jpg
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Lunt 50THa solar scope
Lunt CaK B12 module
SW Evostar 72ED for full disk imaging
Sol'Ex SHG
80mm f/7 achromat used with pressure tune module off the Lunt with Lunt 100mm ERF
Bresser 90 f/10 achromat used for CaK & white light imaging & PST mod with Lunt 100mm ERF
Coronado SM40 (pre Meade) and BF10 filter set + SMII 60 richview DS etalon
Skywatcher EQ6 Pro and Ioptron CEM60
Then there is all the night kit which is far too long a list to write up here! :lol:
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Re: ND filter and Herschel wedge comparison

Post by thesmiths »

That's a really good verification of what I had heard from my friend Jean-Francois Pettit. Hoya are very good filters but I guess they have a fairly significant amount of wavefront error.

As I understand it, a typical wedge polarises the light somewhat on reflection and the grating also has some polarisation preference so proper orientation can give a little extra light. Apparently, the Lacerta wedge, because it operates at the Brewster angle, produces very highly polarised illumination and therefore effectively gives a brighter image when properly used with a grating.

I took a slightly different approach by making my own slits on a fused quartz substrate which means I don't need either a wedge or an ND filter, only a UV/IR cut filter (which lowers the total power input by about 60%).


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Re: ND filter and Herschel wedge comparison

Post by Montana »

Wow!! the Herschel wedge is so much clearer!! I also much prefer this processing as it is much more natural, I hope you stick with this processing routine :)

:bow :hamster:

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Re: ND filter and Herschel wedge comparison

Post by Alun_H »

thesmiths wrote: Sun Nov 13, 2022 6:16 pm
As I understand it, a typical wedge polarises the light somewhat on reflection and the grating also has some polarisation preference so proper orientation can give a little extra light. Apparently, the Lacerta wedge, because it operates at the Brewster angle, produces very highly polarised illumination and therefore effectively gives a brighter image when properly used with a grating.

On his Sol'Ex website Christian Buil states that the Lacerta is the best wedge to use but because of its high stance may not come to focus on all scopes. He also states that the TS wedge has an absorbing polarizing treatment on the glass and causes loss of light. Now I am wondering whether or not the Scopium prism has any coatings as such as I struggle to keep the exposure times down when in the CaK line,although visually looking at the glass I cannot see any signs of a coating,maybe time for an upgrade :)


Lunt 50THa solar scope
Lunt CaK B12 module
SW Evostar 72ED for full disk imaging
Sol'Ex SHG
80mm f/7 achromat used with pressure tune module off the Lunt with Lunt 100mm ERF
Bresser 90 f/10 achromat used for CaK & white light imaging & PST mod with Lunt 100mm ERF
Coronado SM40 (pre Meade) and BF10 filter set + SMII 60 richview DS etalon
Skywatcher EQ6 Pro and Ioptron CEM60
Then there is all the night kit which is far too long a list to write up here! :lol:
More images on view on my Flickr page...https://www.flickr.com/photos/alun_h/
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Re: ND filter and Herschel wedge comparison

Post by Alun_H »

Montana wrote: Mon Nov 14, 2022 12:17 pm Wow!! the Herschel wedge is so much clearer!! I also much prefer this processing as it is much more natural, I hope you stick with this processing routine :)
Definitely a difference between the two which I hadn't notice in my earlier scans with the ND filter. In regards the processing I reckon I will leave the SHG's as is from now on,if the raw image is good enough to that is! :D

Alun


Lunt 50THa solar scope
Lunt CaK B12 module
SW Evostar 72ED for full disk imaging
Sol'Ex SHG
80mm f/7 achromat used with pressure tune module off the Lunt with Lunt 100mm ERF
Bresser 90 f/10 achromat used for CaK & white light imaging & PST mod with Lunt 100mm ERF
Coronado SM40 (pre Meade) and BF10 filter set + SMII 60 richview DS etalon
Skywatcher EQ6 Pro and Ioptron CEM60
Then there is all the night kit which is far too long a list to write up here! :lol:
More images on view on my Flickr page...https://www.flickr.com/photos/alun_h/
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Re: ND filter and Herschel wedge comparison

Post by marktownley »

An enlightening comparison Alun!

Like Alexandra says, leave the processing as it, it works great!


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Re: ND filter and Herschel wedge comparison

Post by Alun_H »

Thank you Aaron,Mark. Going to have to eventually start stacking the images to clean them up noise wise but will not do any processing such as sharpening etc.


Lunt 50THa solar scope
Lunt CaK B12 module
SW Evostar 72ED for full disk imaging
Sol'Ex SHG
80mm f/7 achromat used with pressure tune module off the Lunt with Lunt 100mm ERF
Bresser 90 f/10 achromat used for CaK & white light imaging & PST mod with Lunt 100mm ERF
Coronado SM40 (pre Meade) and BF10 filter set + SMII 60 richview DS etalon
Skywatcher EQ6 Pro and Ioptron CEM60
Then there is all the night kit which is far too long a list to write up here! :lol:
More images on view on my Flickr page...https://www.flickr.com/photos/alun_h/
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Re: ND filter and Herschel wedge comparison

Post by christian viladrich »

The difference is quite significant !

This is not totaly surprising since Hoya filters are just photographic filters. There are not lambda/4 P-V optical plates.


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Re: ND filter and Herschel wedge comparison

Post by Alun_H »

This has put me off using the filter for this type of work and will remain with the wedge from now on,Sol'Ex is light enough to not warrant any flexure
problems.

Alun


Lunt 50THa solar scope
Lunt CaK B12 module
SW Evostar 72ED for full disk imaging
Sol'Ex SHG
80mm f/7 achromat used with pressure tune module off the Lunt with Lunt 100mm ERF
Bresser 90 f/10 achromat used for CaK & white light imaging & PST mod with Lunt 100mm ERF
Coronado SM40 (pre Meade) and BF10 filter set + SMII 60 richview DS etalon
Skywatcher EQ6 Pro and Ioptron CEM60
Then there is all the night kit which is far too long a list to write up here! :lol:
More images on view on my Flickr page...https://www.flickr.com/photos/alun_h/
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Re: ND filter and Herschel wedge comparison

Post by marktownley »

I'd be interested to see a a write up and some pics of the setup you're using Alun, apologies if i've missed this already :lol:


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Re: ND filter and Herschel wedge comparison

Post by Alun_H »

marktownley wrote: Thu Nov 17, 2022 6:30 am I'd be interested to see a a write up and some pics of the setup you're using Alun, apologies if i've missed this already :lol:
No problem Mark here are a few photos of the rig,SW Evostar 72ED doublet apo with 3D printed Sol'Ex SHG which I have wrapped in heat deflecting cloth which is used in engine bays etc. I also found that there is a light leak in the PETG tube between the focuser and the units body so wrapped that with Washi paper (a Japanese sticky tape in matt black,brilliant stuff for making anything light tight). Must add I didn't print the Sol'Ex kit, I ordered it from Azure3DPrint in France it arrived fully assembled with threaded inserts and bolts leaving me to add the optical kit from Shelyak.

Alun
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Lunt 50THa solar scope
Lunt CaK B12 module
SW Evostar 72ED for full disk imaging
Sol'Ex SHG
80mm f/7 achromat used with pressure tune module off the Lunt with Lunt 100mm ERF
Bresser 90 f/10 achromat used for CaK & white light imaging & PST mod with Lunt 100mm ERF
Coronado SM40 (pre Meade) and BF10 filter set + SMII 60 richview DS etalon
Skywatcher EQ6 Pro and Ioptron CEM60
Then there is all the night kit which is far too long a list to write up here! :lol:
More images on view on my Flickr page...https://www.flickr.com/photos/alun_h/
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Re: ND filter and Herschel wedge comparison

Post by daoma2 »

Hi
You may want to take a look at the Nisi ND IR filter, as far as i know they are the best when it comes to extreme nd filter, they have a 15 stops version which should work fine. The hoya might be a bit older and cheaper, but from your samples still does the job without further processing. I suspect that the Nisi + stacking + postprocess would look very similar to the prism, while having the advantage that it can be used on camera lenses and for landscapes imaging too.


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